The present invention relates to a driving force distribution control system for controlling a transfer of a 4WD system.
The above-mentioned U.S. application Ser. No. 830,015 corresponding to Japanese patent provisional publication No. 61-191431 (Application No. 60-33286) shows a conventional example of the driving force distribution control system. This control system is arranged to vary the driving force distribution toward the 2WD state when the front wheel speed is higher than the rear wheel speed, and toward the 4WD state when the rear wheel speed is higher than the front wheel speed, in order to prevent both tight corner braking and wheel spin without causing an abrupt change in the steering characteristic.
Another conventional example is shown in Japanese Utility Model provisional publication No. 61-143927(Application No. 60-28503). A driving force distribution control system of this example is arranged to vary the driving force distribution gradually from the 2WD state toward the 4WD state during braking in order to improve the braking ability of the vehicle during engine braking without changing the steering characteristic abruptly.
However, the former system is arranged to vary the driving force distribution to the 2WD state when the front wheel speed is higher than the rear wheel speed. Therefore, the 2WD state is selected even when the accelerator is off, so that the braking torque due to engine braking is alloted only to the two wheels. Consequently, this conventional system tends to increase a tire slip ratio especially on a road surface of a low friction coefficient, and degrade the directional stability of a vehicle in straight ahead driving.
The latter system is arranged to vary the driving distribution to the 4WD state independently of the vehicle speed when the accelerator is off, and the front wheel speed is higher than the rear wheel speed. Therefore, this conventional system is unable to discriminate whether the excess of the front wheel speed over the rear wheel speed is caused by slip due to engine braking or by cornering. That is, this conventional system cannot prevent tight corner braking sufficiently.